CVE-2021-36403Network attack vector

Phishing via concealed email message link

Published Mar 6, 2023 · Updated Mar 7, 2025

HTML injection in Moodle LMS 3.11, 3.10 through 3.10.4, and 3.9.7 and earlier allows remote attackers to hide message links in email. The messaging email-notification path renders attacker-controlled HTML without protecting the final system-generated link, so crafted markup can conceal that link. The affected path requires an HTML-capable message to trigger an email notification, enabling deceptive email presentation rather than code execution or data disclosure.

CVSS severity5.3
Medium
EPSS probability0.53%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTML injection in Moodle LMS 3.11, 3.10 through 3.10.4, and 3.9.7 and earlier allows remote attackers to hide message links in email. The messaging email-notification path renders attacker-controlled HTML without protecting the final system-generated link, so crafted markup can conceal that link. The affected path requires an HTML-capable message to trigger an email notification, enabling deceptive email presentation rather than code execution or data disclosure.

The record

CVE
CVE-2021-36403
Published
Mar 6, 2023
Updated
Mar 7, 2025
Vendor
Moodle
Product
Moodle LMS
Classifications
CWE-912, T1566
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 6, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 7, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=3.11, 3.10 to 3.10.4, 3.9 to 3.9.7 and earlier unsupported versions

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Moodle · Moodle LMSversion=3.11, 3.10 to 3.10.4, 3.9 to 3.9.7 and earlier unsupported versions

Published CVSS scores

5.3CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-912Hidden Functionality
T1566Phishing

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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